Friction stir interlocking of dissimilar materials

ABSTRACT

A method for solid state joining of dissimilar materials using a friction stir welding device wherein a pin is inserted through an aperture defined in a first material and a second material to hold the materials together and then held in place by friction stir welding a portion of the pin to a material adjacent said pin, or by friction stir welding a cap or plug that holds the pin in place to the adjacent material. The result is a connection or join wherein the central portion of the pin is not friction stir welded but the portions holding the pin in place (the ends or caps) generally are.

CLAIM TO PRIORITY

This application claims priority from and is a divisional application ofapplication Ser. No. 15/794,687 filed Oct. 26, 2017 entitled FrictionStirring Interlocking of Dissimilar Materials, the contents of which areherein incorporated by reference.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with Government support under ContractDE-AC0576RL01830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

The invention generally relates to methods for joining dissimilarmaterials and more particularly to connections between dissimilar metalshaving different melting points.

A world of rising energy necessitates approaches for reducing the amountof energy needed to perform standard tasks. Among approaches underdevelopment are lighter, more fuel efficient vehicles. Reducing theweight of vehicles can be accomplished in a variety of ways includingreplacing heavier steel regions with lighter weight materials. However,difficulty has arisen in attempting to find ways to robustly joindissimilar materials in a way that provides the needed strength andresiliency that exists in structures that are made from the samematerial. Preferably, and in some instances by requirement, these seamsand interconnects must be welded together. Welding is fairly straightforward when the two materials have similar melting points but becomesmore and more difficult when the materials have vastly different meltingpoints or other characteristics.

Joining materials such as steel to aluminum, titanium, magnesium,copper, or any combination thereof, has proved difficult for a varietyof reasons. The prior art generally teaches that when these materialsare joined that the temperatures must be maintained generally low so asto prevent the formation of brittle intermetallic compounds, which aregenerally believed to cause the welds to be brittle and fail. Most priorart methodologies for joining dissimilar materials have focused ongetting rid of these brittle intermetallic portions. However, the workarounds have generally proven to have negative side effects such as costand complexity and in many instances simply do not provide an acceptablesolution.

Hence what is needed is a process for forming high strength jointsbetween dissimilar materials in ways that a simpler cheaper and moreeffective than the current methodologies. The present invention is asignificant step forward in addressing these needs.

Additional advantages and novel features of the present invention willbe set forth as follows and will be readily apparent from thedescriptions and demonstrations set forth herein. Accordingly, thefollowing descriptions of the present invention should be seen asillustrative of the invention and not as limiting in any way.

SUMMARY

The present disclosure provides a method for solid state joining ofdissimilar materials using a friction stir welding device wherein a pinis inserted through an aperture defined in a first material and a secondmaterial to hold the materials together and then held in place byfriction stir welding a portion of the pin to a material adjacent saidpin, or by friction stir welding a cap or plug that holds the pin inplace to the adjacent material. The result is a connection or joinwherein the central portion of the pin is not friction stir welded butthe portions holding the pin in place (the ends or caps) generally are.

The process makes possible the connection and interconnection of avariety of materials including embodiment wherein the combinations suchas steel and aluminum, magnesium and aluminum or magnesium and steel ora metal and a non-metal such as aluminum and a carbon reinforced polymercan be interconnected. In some embodiments the first material isconfigured in to a C-shape and said second material is embodied in aninsert, however a variety of other configurations are also possible andcontemplated for obtaining this pin held inter connection.

This process and configuration finds application in a variety of fieldsincluding but not limited to: transportation vehicle manufacture,electronic product manufacturing, construction trades manufacturing andother applications. According the descriptions provided previously andthe detailed descriptions here after should be seen as illustrative andnot limiting.

Various advantages and novel features of the present invention aredescribed herein and will become further readily apparent to thoseskilled in this art from the following detailed description. In thepreceding and following descriptions, only the preferred embodiment ofthe invention have been shown and described, by way of illustration ofthe best mode contemplated for carrying out the invention. As will berealized, the invention is capable of modification in various respectswithout departing from the invention. Accordingly, the drawings anddescription of the preferred embodiment set forth hereafter are to beregarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a detailed cut through view of one example of the presentdisclosure.

FIGS. 2(a)-2(d) show various alternative applications for connectingdissimilar materials.

FIGS. 3(a)-(d) show a step wise process of one embodiment of thedisclosure.

FIG. 4 shows another embodiment of the disclosure

FIG. 5 shows a cut away of a joint described in the present disclosure

FIG. 6 shows the results of testing performed in various describedapplications.

FIGS. 7(a)-(d) shows a step wise process of another embodiment.

FIG. 8 shows another embodiment including a T-joint configuration

DETAILED DESCRIPTION

The following description includes examples of various embodiments ofthe present disclosure. It will be clear from this description of thatthe invention is not limited to these illustrated embodiments but alsoincludes a variety of modifications and embodiments thereto. Therefore,the present description should be seen as illustrative and not limiting.There is no intention in the specification to limit the invention to thespecific form disclosed, but, on the contrary, the invention is to coverall modifications, alternative constructions, and equivalents fallingwithin the spirit and scope of the invention as defined in the claims.

Friction Stir Interlocking (FSI) is a new methodology for joiningdissimilar materials such as lightweight metals to composites, thermosetplastics, or other non-metallic materials. Metals with vastly differentmelting temperatures which cannot be joined by conventional welding canalso be joined by FSI. Currently the state-of-the-art for joining Mg andAl to carbon fiber (CF) and other non-metal is conventional mechanicalfastening and adhesive bonding. Solid-phase approaches such as FrictionStir Welding, Scribe, Riveting, Pillaring and Spot Welding are all beinginvestigated but face significant challenges with joint strength,fatigue, damaging the carbon fiber materials, slow process speed andgalvanic corrosion. The FSI process described herein provides solutionsto many of these issues.

In one embodiment shown in FIG. 1, a method for joining Mg and Al tonon-metals is shown. Referring first to FIG. 1. Here, pins 12 that matchthe material of the first sheet 10 (in this case a metal such as Mg orAl) 10 are inserted up through holes 14 cut in the second material (inthis case a non-metal such as a carbon fiber or carbon reinforced carboncomposite 11 and first sheet 10. These pins 12 are designed to be flushwith the top of the sheet when inserted. (While in this particulararrangement the pins are designed to be flush it is to be understoodthat this arrangement is only exemplary and that pins may be variouslyarranged and designed to extend above or below the surfaces of the topor bottom sheets depending upon the needs of the user.) A speciallydesigned FSW tool 5 then traverses the joint 7 and welds the pins 12 tothe sheet 10 to complete the joint 7. The large hydrostatic pressure inthe plasticizing metal during welding will fill any small tolerancegaps, between the pin 12 and the non-metal material which defines thehole 14 through which the pin 12 was inserted. In some instances athermally activated adhesive film 16 can be applied between themetal-non-metal interface 13 prior to welding to improve joint 7strength. The film 16 will also serve as a barrier to galvanic corrosionby sealing against electrolyte imbibition into the joint interfaces. Thejoint could certainly be made without the added step of an adhesive film16 if desired. For the example in FIG. 3 an arrangement is providedwherein embedded bar inserts are shown running the entire course of theweld or joint 7 or smaller inserts to accommodate spot or stitch weld.

In a variety of other embodiments are variety of other shapes, patternsand cross sections for the various pins 12 and corresponding holes 14through which they can be inserted. As a friction stir process, numerousinterlocks can be created quickly and uniformly, in a single pass,offering reduced cost and improved process efficiency compared toconventional metal-to-non-metal fasteners. Galvanic corrosion betweenthe metal fasteners and carbon fibers can be nearly eliminated usingFSI. In many instance the short process time (a few seconds) and lowprocess temperature (as low as 250° C. for Mg) make this friction stirinterlocking FSI approach attractive for joining Mg and Al to carbonfiber CF without substantially degrading the CF material properties.

In another embodiments of the invention (shown in the various FIG. 2arrangements) Metal inserts 20 of various configurations can be placedwithin a non-metal materials, such as carbon fiber reinforced polymer(CFRP) or carbon fiber reinforced composite (CFRC) during the typicalCFRC or CFRP production processes. Once embedded within the non-metalmaterials, these metal materials can be subsequently friction stirwelded to Mg, Al or other metal materials. As such, a metallurgical bondis formed between the sheet and insert, and joint strength is governedby the chemical bonding and mechanical interlocking between the metalinsert and the non-metal material (CFRP). With this process, mechanicalinterlocks are created without disrupting the carbon fibers or matrix.In one exemplary arrangement an aluminum insert is embedded into acarbon fiber composite during the fabrication of the composite. Thecarbon fiber composite is then injection molded around the insert tocreate an assembly such as the ones shows in FIG. 2.

In other embodiments the plate of insert material such as Mg. or Al canbe placed over a CFRP or CFRC assembly and then friction stir wieldedalong a weld path joining the plate to the insert embedded duringfabrication of the CFRP or CFRC. Linear inserts such as these that areshown are can be advantageous because they provide a continuous jointalong the dissimilar interface. Spot welding inserts using similarmethods could also be embedded during the manufacturing of the carbonfiber composite. Forming carbon fiber composites around inserts allowfor resign to adhere to the insert and allow for mechanicalinterlocking. This method allows for faster FSW processes and reducesheat input into carbon fiber.

In other variations, magnesium overcasting can be performed using any ofa variety of metals such as Mg or Al and alloys thereof. In oneparticular instance a high-pressure die cast (HPDC) Mg alloy is directlycast over a short section of the CFRC component such that the carbonfiber reinforced composite CFRC section is completely embedded withinthe Mg casting to create a strong mechanical interlocking joint.Embedded inserts may also be used to enhance joint strength. AlthoughCFRC will in general burn/decompose easily at the temperaturecorresponding to the melting point of Mg alloys (T>600 C), rapidsolidification of molten Mg (i.e. within 1-2 seconds) and subsequentcooling during HPDC will sufficiently limit the surface decomposition ofthe bulk CFRP to form a robust mechanical joint.

The advantage of the Mg-CFRC overcasting method is that it avoids theneed for machining or disturbance of the casting or CFRC while enablingjoint geometries that are otherwise cumbersome to machine or notfeasible by conventional mechanical fastening methods. While challengesmay arise in creating a Mg-CFRC joint by overcasting because of theburn-off/thermal decomposition of the CFRC composite when it comes incontact with molten Mg and also during subsequent cooling of thesolidified casting. This challenge can be addressed in two ways: 1) theMg solidification rate is controlled to minimizing the duration forwhich the CFRC is exposed to temperature above its thermal decompositiontemperature and 2) temperature-resistant coatings (e.g. graphite, boronnitride, etc.) on the CFRP are used to prevent erosion via directcontact with flowing molten metal.

In other circumstances, various other interlocking configurations andapplications are shown. FIG. 3 shows an arrangement wherein, a block ofa first material 13 such as aluminum is machined to produce a series ofapertures or cutouts 14 dimensioned to align with apertures in aninserted second material 15 as well as various pins 12 which are to beentered through these various apertures 14. These sections in the firstmaterial are configured to align with a corresponding set of apertures14 in a second material, A pin 12, typically made of the same materialas the block and having a length sufficient to extend through the block13 and to match the top and bottom surfaces of the block is theninserted through the openings and a friction stir welding device is thenused to plasticize and weld the two ends of the pin 12 into the block 13and within block 15. When this happens the first block 13 and pin 12materials plasticize and fill gaps, however the distance between thesecond material 15 and the FSW tool pin tip prevent plasticization ofthe entire pin 12 and also prevents formation of intermetallic featureswithin the block 13. This will provide a rapid, cost-effective,repeatable process. Such a process could be an enabling technology forlight weighting of automotive components or any application where robustjoints are needed for example between steel, Mg, Al and various othermetals and non-metals.

In one example shown in FIG. 4, a welded plate formed according to thisprocess is shown. In testing several sections of this welded plate wereremoved and subjected to tensile testing. FIG. 5 shows one of thesesections that was cut through the center portion of a pin is showninterlocking between the aluminum and steel. The location is the toolpin path where the pin is processed as indicated, while the pin withinthe steel is not processed. A plot showing this data is provided in FIG.6 where the lower two curves are for aluminum pins, processed withdifferent weld speeds and penetration depths.

In another embodiment of the invention the aluminum pin 12 is replacedwith a steel pin 12 and the approach is to lock the steel pin into thealuminum via friction stir welding. This is done by FSW through aluminumplugs 22 inserted on each side of the steel pin 12 as shown in FIG. 7.This approach has particular application to large scale applicationssuch as bridges, towers and other large structures where joint strengthand corrosion is currently a limitation for aluminum to steelconnections. Unlike conventional bolt/nut fastening, this approach sealsthe fastener from electrolyte penetration which occurs at the head andnut end of conventional fasteners. The upper curve in FIG. 6 shows thestrength advantage of the concept described in FIG. 7.

FIG. 8 shows another arrangement (a T-shape) wherein plates of a firstmaterial (10, 10′) such as Mg or Al are connected to a plate 11 of asecond material such as steel by running pins 12 made of the firstmaterial through apertures 14 contained in the plates and friction stirwelded into place so as to form a single unitary joint 7. The presentinvention provides a variety of advantages over the prior art andprovides the ability to connect dissimilar materials and enablestructures and arrangements that are not currently available.

While various preferred embodiments of the invention are shown anddescribed, it is to be distinctly understood that this invention is notlimited thereto but may be variously embodied to practice within thescope of the following claims. From the foregoing description, it willbe apparent that various changes may be made without departing from thespirit and scope of the invention as defined by the following claims.

What is claimed is:
 1. A method for solid state joining of dissimilarmaterials using a friction stir welding device comprising the steps of:inserting an insert within an aperture defined in a first material and asecond material, and friction stir welding a portion of said insert to amaterial adjacent to said pin.
 2. The method of claim 1 wherein saidfirst material is aluminum.
 3. The method of claim 1 wherein said secondmaterial is steel.
 4. The method claim 1 wherein said second material isa carbon reinforced composite.
 5. The method of claim 1 wherein saidfirst material is configured in to a C-shape and said second material isembodied in an insert configured to fit within said C-shaped material.6. The method of claim 4 wherein said insert is a pin has two ends andis friction stir welded in each end to the first material.
 7. The methodof claim 5 wherein said first material is aluminum and said secondmaterial is steel.
 8. The method of claim 5 wherein said first materialis steel and said second material is aluminum.
 9. A method for solidstate joining of dissimilar materials using a friction stir weldingdevice comprising the steps of: inserting a pin through an aperturedefined in a first material and a second material; capping at least oneend of pin with a cap material; and friction stir welding said cap to amaterial adjacent said cap.
 10. The method of claim 9 wherein said firstmaterial is aluminum.
 11. The method of claim 9 wherein said secondmaterial is steel.
 12. The method claim 9 wherein said second materialis a carbon reinforced composite.
 13. The method of claim 9 wherein saidfirst material is configured in to a C-shape and said second material isembodied in an insert.
 14. The method of claim 9 wherein said firstmaterial is aluminum and said second material is steel.
 15. The methodof claim 9 wherein said first material is steel and said second materialis aluminum.
 16. A method of joining a first metal material to a carbonreinforced composite comprising the steps of overcasting the metalmaterial over the carbon reinforced composite material.
 17. A method ofjoining a metal material to a carbon reinforced composite comprising thesteps of inserting a metal insert within the carbon reinforced compositeduring fabrication of the carbon reinforced composite and then frictionstir welding a second piece of metal to the metal insert or inserts thatwere embedded during the fabrication of the said composite.
 18. A jointcomprising: a first material connected to a second material by a pinpassing through said first material and said second material, said pinheld in place by friction stir welded portions.
 19. The joint of claim16 wherein the ends of the pin are friction stir welded to a materialadjacent the end of said pin.
 20. The joint of claim 16 where in the pinis held in place by at least one cap that is friction stir welded to thematerial adjacent said cap.